1. Field of the Invention
The present invention relates to a sound image control system, and more particularly, to a sound image control system controlling a sound image localization position by reproducing an audio signal from a plurality of loudspeakers.
2. Description of the Background Art
In recent years, a multichannel signal reproduction system typified by a DVD has become prevalent. However, housing conditions often do not allow for the installation of five or six loudspeakers. Therefore, a sound image control system using a so-called virtual reproduction method, which realizes virtual reproduction of a surround signal with Lch and Rch loudspeakers, has been developed.
Also, especially in a sound image control system for car audio equipment, the placement of loudspeakers in a narrow inside space of a vehicle is limited due to considerable influences of reflection, reverberation, and standing waves. In such an arrow space as the inside of a vehicle, it is conventionally rather difficult to freely localize a sound image. However, there is still a strong demand to localize vocals, etc., included in music in the front center of a passenger. In order to satisfy the above-described demand, a sound image control system as described below is in the process of being developed.
Hereinafter, with reference to a drawing, the conventional sound image control system is described. FIG. 47 is an illustration showing the structure of the conventional sound image control system. In FIG. 47, the sound image control system installed in a vehicle 601 includes a sound source 61, a signal processing section 62, an FR loudspeaker 621 placed on the right front door of the vehicle 601, and an FL loudspeaker 622 placed on the left front door of the vehicle 601. The signal processing section 62 has control filters 63 and 64.
An operation of the sound image control system shown in FIG. 47 is described below. A signal from the sound source 61 is processed in the signal processing section 62, and reproduced from the FR loudspeaker 621 and the FL loudspeaker 622. The control filter 63 controls an Rch signal from the sound source 61, and the control filter 64 controls an Lch signal from the sound source 61. The signal processing section 62 performs signal processing so that sound from the FR loudspeaker 621 is localized in a position of a target sound source 631 and sound from the FL loudspeaker 622 is localized in a position of a target sound source 632. Specifically, the control filters 63 and 64 of the signal processing section 62 are controlled as follows. That is, assume that a center position (a small cross shown in FIG. 47) of a listener A is a control point, a transmission characteristic from the FR loudspeaker 62 to the control point is FR, a transmission characteristic from the FL loudspeaker 622 to the control point is FL, a transmission characteristic from the target sound source 631 to the control point is G1, and a transmission characteristic from the target sound source 632 to the control point is G2, characteristics HR and HL of the respective control filters 63 and 64 in the signal processing section 62 are represented by the following expressions.HR=G1/FRHL=G2/FL
The characteristics (HR and HL) satisfying the above-described expressions allow the FR loudspeaker 621 to be controlled so as to reproduce sound in the position of the target sound source 631, and the loudspeaker 622 to be controlled so as to reproduce sound in the position of the target sound source 632. As a result, a center component common to the Lch signal and the Rch signal is localized between the virtual target sound sources 631 and 632. That is, the listener A localizes a sound image in a position of a front target sound source 635.
However, the conventional system shown in FIG. 47 has only one control point. As a result, the difference between the right and left ears, which is the mechanism of perception, is not controlled, thereby having a limited sound image localization effect. Furthermore, most sound image control systems in practical use only correct a time lag between the FR loudspeaker 621 and the FL loudspeaker 622, thereby not actually realizing the virtual target sound sources 631 and 632.
As a sound image control system for home use, on the other hand, a sound image control system performing sound image control by setting both ears as control points has been developed. However, in the above-described sound image control system, the number of control points is assumed to be two, that is, both ears of a single listener are assumed to be the control points. Therefore, the above-described sound image control system does not concurrently perform sound image control for both ears of two listeners.